This invention relates to document scanning systems and, more particularly, to circuitry responsive to light reflected from a scanned document to provide output signals representative of the information content of the scanned document.
This invention finds particular utility in a facsimile transmitter wherein an information bearing document is scanned by a source of light and the light reflected from the document is converted into an information bearing electrical signal suitable for transmission over a communication channel to a facsimile receiver which then converts the information bearing electrical signal into an information bearing document which is a facsimile, or remote reproduction, of the original scanned document. The prior art is replete with examples of facsimile transmitters which utilize a light source, a photodetector, and a means for creating relative motion in two dimensions between the light source and photodetector as a unit, on the one hand, and the copy to be transmitted, on the other. This provides for raster scanning of the original document. One conventional way of accomplishing this scanning is to mount the document on a drum, rotate the drum with the document wrapped thereabout, and move the light source and photodetector along a line parallel to the axis of the drum. This results in a helical scanning pattern. Another means of accomplishing the scanning of a document is to slowly advance the document past a line-like scanning station, the photodetector being fixed to capture light reflected from the document. In all of these prior art systems, the light impinging on the photodetector is converted thereby into an electrical signal, which is then amplified to suitably modulate a carrier signal for transmission over a communication channel to the remote receiver. In all these systems, it is desirable to be able to transmit the information content of a document independent of the background density of the document without transmitting a background level signal that would deteriorate the quality of the reproduced image.
A problem with prior art document scanners arises if the platen back is darker than the background of a scanned document. An unaesthetic appearance of the reproduced copy results when a narrow document is scanned. In addition, a problem of "looking through" onionskin documents arises which results in high video noise levels, transmission of water marks, etc. However, if the platen back is lighter than the background of a scanned document, the automatic background control circuitry is upset by initial runs of white in the event a non-white document is imperfectly registered.
It is therefore an object of this invention to provide an amplifier for use in a document scanning system.
It is a further object of this invention to provide an amplifier with automatic background control circuitry which adjusts to the background level of the document to be transmitted.
It is yet another object of this invention to provide circuitry which compensates for the inherent dark current of the photodetector so that a signal representative only of the information content of the document is provided.
It is still another object of this invention to provide circuitry which responds quickly to a lightening of the background of a scanned document but responds slowly to a darkening of the background of a scanned document.
Another object of this invention is to provide an improved platen back which prevents the automatic background control circuitry from being upset by initial runs of white in the event of an imperfectly registered non-white document.
Other objects of the present invention will be evident from the description hereinafter presented.